Hot rolling and cold rolling are both processes for shaping steel plates or profiles. They have a significant impact on the structure and properties of the steel. So, what are the differences between them? Today, our editor will guide you to explore this. 

 

 

The rolling of steel mainly relies on hot rolling. Cold rolling is usually only used for producing small-sized steel sections and thin plates and other precision-sized steel products. 

Common cold and hot rolling conditions of steel: 

Wire material: Diameter 5.5 - 40 millimeters, in coil form, all are hot-rolled materials. After cold drawing, they become cold-drawn materials. 

Round steel: Apart from the precision-sized bright material, it is usually hot-rolled. There are also forged materials (with forging marks on the surface). 

Steel strip: Both hot-rolled and cold-rolled types are available. Cold-rolled materials are generally thinner. 

Steel plates: Cold-rolled plates are generally thinner, such as those used in automobiles. Hot-rolled medium-thick plates are more common, and their appearance is significantly different. 

Angle steel: All are hot-rolled. 

Steel pipes: Both hot-rolled and cold-drawn welding methods are used. 

Channel steel and H-shaped steel: Hot rolled. 

Rebar: Hot-rolled material. 

Hot rolling 

By definition, steel ingots or billets are difficult to deform at room temperature and are not easy to process. They are generally heated to 1100℃ to 1250℃ for rolling, and this rolling process is called hot rolling. 

 

 

The termination temperature of hot rolling is generally between 800℃ and 900℃. Afterwards, it is usually cooled in the air, thus the hot-rolled state is equivalent to normalizing treatment. 

 

 

Most of the steel is rolled by the hot-rolling method. The steel delivered in the hot-rolled state, due to the high temperature, forms a layer of iron oxide scale on the surface, thus having certain corrosion resistance and can be stored outdoors. 

 

 

However, this layer of iron oxide also makes the surface of hot-rolled steel rough and causes significant size variations. Therefore, for steel with a smooth surface, precise dimensions, and excellent mechanical properties, it is necessary to use hot-rolled semi-finished products or finished products as raw materials for cold rolling production. 

 

 

Advantages:

 

 

 

It has a fast forming speed, high output, and does not damage the coating. It can be made into various cross-sectional forms to meet the requirements of the usage conditions. Cold rolling can cause significant plastic deformation in steel, thereby increasing the yield point of the steel. 

 

 

Disadvantages: 

 

 

Although no hot plastic compression was involved during the forming process, residual stresses still existed within the cross-section, which inevitably had an impact on the overall and local buckling characteristics of the steel. 

 

 

2. The typical shape of cold-rolled steel sections is an open cross-section, which results in a relatively low free torsional stiffness of the section. During bending, torsion is prone to occur; during compression, buckling due to both bending and torsion is likely to happen, and the torsional resistance is poor. 

 

 

3. The cold-rolled formed steel has a relatively thin wall thickness, and there is no thickening at the corner where the plates meet. Therefore, it has a weak ability to withstand localized concentrated loads. 

 

 

Cold rolling 

 

 

Animation of the production process of the cold-rolled galvanized line 

 

 

Cold rolling refers to a rolling method in which steel is compressed under the pressure of rollers at room temperature to change its shape. Although the processing process may cause the steel plate to heat up, it is still called cold rolling. To be more specific, cold rolling uses hot-rolled steel coils as raw materials, undergoes acid washing to remove the oxide layer, and then undergoes pressure processing. The finished product is a rolled hard coil. 

 

 

Generally, cold-rolled steel such as galvanized and color-coated steel must undergo annealing, so it has better plasticity and elongation, and is widely used in industries such as automobiles, home appliances, and hardware. The surface of cold-rolled plates has a certain degree of smoothness, and it feels relatively smooth when touched by hand, mainly due to the effect of acid washing. The surface smoothness of hot-rolled plates generally does not meet the requirements, so hot-rolled steel strips need to be cold-rolled. Also, the thinnest thickness of hot-rolled steel strips is usually 1.0mm, while cold-rolled steel can reach 0.1mm. Hot rolling is the rolling process above the crystallization temperature point, while cold rolling is the rolling process below the crystallization temperature point. 

 

 

The deformation of the steel shape during cold rolling is a continuous cold deformation process. The cold work hardening caused by this process leads to an increase in the strength and hardness of the rolled hard coils, while the ductility and plasticity indicators decrease. 

 

 

For the end-user, cold rolling deteriorates the stamping performance, and the product is suitable for simple-shaped parts. 

 

 

Advantages:

 

 

 

It can damage the casting structure of the steel ingot, refine the crystal grains of the steel, and eliminate the defects in the microstructure, thereby making the steel structure denser and improving its mechanical properties. This improvement is mainly manifested in the direction of rolling, so that the steel is no longer an isotropic body to a certain extent; the bubbles, cracks and porosity formed during pouring can also be welded together under the action of high temperature and pressure. 

 

 

Disadvantages: 

 

 

After hot rolling, the non-metallic inclusions (mainly sulfides, oxides and silicates) within the steel are compressed into thin sheets, resulting in a layering phenomenon. This layering significantly deteriorates the tensile properties of the steel along its thickness direction, and there is a possibility of inter-laminar tearing during the shrinkage of the weld seam. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much greater than the strain caused by loads. 

 

 

2. Residual stress caused by uneven cooling. Residual stress refers to the internal stress that is in equilibrium without external force. All types of hot-rolled steel sections have such residual stress. Generally, the larger the cross-sectional dimensions of the steel sections, the greater the residual stress. Although residual stress is self-balancing, it still has certain effects on the performance of steel components under external forces. It may have adverse effects on aspects such as deformation, stability, and fatigue resistance. 

 

 

Summary: 

 

 

The main difference between cold rolling and hot rolling lies in the temperature during the rolling process. "Cold" refers to normal temperature, while "hot" refers to high temperature. 

 

 

From the perspective of metallurgy, the boundary between cold rolling and hot rolling should be determined by the recrystallization temperature. That is, rolling below the recrystallization temperature is considered as cold rolling, while rolling above the recrystallization temperature is classified as hot rolling. The recrystallization temperature of steel ranges from 450℃ to 600℃. 

 

 

The main difference between hot rolling and cold rolling is: 

 

 

1. Appearance and surface quality 

 

 

Since the cold plate is obtained by the cold rolling process of the hot plate, and the cold rolling process also involves some surface finishing operations, the surface quality (such as surface roughness, etc.) of the cold plate is better than that of the hot plate. Therefore, if there are high requirements for the coating quality (such as painting) in the subsequent processing of the product, cold plates are generally chosen. The hot plates are divided into acid-washed plates and non-acid-washed plates. The surface of the acid-washed plate is normal metallic color due to the acid washing process, but the non-acid-washed plate has not undergone cold rolling and thus has a lower surface quality than the cold plate. The non-acid-washed plate usually has an oxide layer, appears dark or has a ferric oxide layer. In simple terms, it is like being "roasted over fire", and if the storage environment is poor, it usually has a bit of rust. 

 

 

2. Performance 

 

 

Generally speaking, in engineering, the mechanical properties of hot plates and cold plates are considered to be the same. Although cold plates undergo certain work hardening during the cold rolling process (though in cases where strict mechanical performance requirements are imposed, special treatment is necessary), cold plates typically have a slightly higher yield strength and a higher surface hardness than hot plates. The exact situation depends on the degree of cold plate annealing. However, regardless of the degree of annealing, the strength of cold plates is higher than that of hot plates. 

 

 

3. Forming Performance 

 

 

Since the performance of the hot and cold plates is not significantly different, the influencing factors of the forming performance mainly depend on the differences in their surface quality. As the surface quality of the cold plates is generally better, in general, for steel plates of the same material, the forming effect of the cold plates is better than that of the hot plates.